Why is my scale inaccurate?

Is your scale suddenly showing a different weight from what you expect, is the value on the display jumping, or do you get different results with the same load? That does not mean the instrument is broken. With the checks below you can rule out most causes yourself, before calibration or repair is needed.

1. Start with zero and tare

Clear the weighing platform completely. Without a load the scale should read zero. If it does not, check whether a tare value is still active, whether something is lying on the platform and whether the platform is touching anything.

A tare that has been left active is one of the simplest causes of a consistently low weight. Zero the scale again and weigh once more. The difference between zeroing and taring is explained in what is tare and how does taring work.

2. Is the scale level?

Precision balances, laboratory balances and analytical balances must stand horizontal and stable. That is why most models have a built-in spirit level and adjustable feet. Check the level again whenever an instrument has been moved.

With a floor scale the surface also counts: a frame that is put under tension by an uneven floor changes the force distribution. The available versions are under floor scales.

3. Vibration and an unstable surface

A scale measures force, and vibrations are force. Forklifts, machines, compressors, conveyor belts, a springy floor or a wobbly workbench all show up as an unstable or fluctuating value.

The finer the readability, the more the environment matters. An analytical balance with a readability of 0.1 mg places very different demands on the set-up from a floor scale that reads in 200 g steps.

4. Temperature, draughts and warm-up time

Temperature changes, air flow and too little warm-up time are immediately noticeable on precise balances. This comes into play when an instrument has just been switched on, comes from a cold room, stands next to an outside door, or directly under the air conditioning, the ventilation or in the sun.

Allow a precise balance to acclimatise and warm up before carrying out critical measurements. With the finest measurements, humidity and static electricity also play a role. For this kind of work there are laboratory balances and analytical balances.

5. Does the scale read the same everywhere on the platform?

Place the same test weight in succession in the centre and at different positions on the platform. The results should be within tolerance of each other.

If the same mass reads clearly differently on one side than in the centre, this may be due to eccentric loading, a mechanical obstruction, an incorrect corner setting or a failing load cell.

6. Is the platform touching anything?

With floor, pallet and built-in weighing systems the platform must be able to move completely freely. If it presses against a frame, a wall, a pipe, a cable or accumulated dirt, a second force path is created and the scale reads too little, too much or unsteadily.

So also look under and around the platform. With a pit scale, material between the edge of the pit and the platform is enough to disturb the measurement.

7. Is the load completely free?

Not every deviation comes from the instrument. A pallet, tank or hopper must not touch anything else while it is being weighed. A hose that pulls, a pipe, a cable or a roller conveyor also transfers force that does not pass through the weighing system.

Place a load entirely on the platform, so that the weight passes only through the measuring system.

8. Has the scale suffered an impact or overload?

A load cell is made for a certain range and a certain maximum load. A falling load or a collision produces a short peak that is much higher than the weight that eventually rests on it.

Think of a pallet that is put down hard, a pallet truck that hits a floor scale, or an instrument that receives a shock during transport. A damaged load cell can still show a plausible value afterwards, while the repeatability, linearity or corner loading are no longer correct. What can be done about it is under scale repair.

9. Check with a known test weight

If you want to establish whether the scale really deviates, use a weight whose mass is reliably known. For a serious check a calibrated weight is better than a random object.

Four things to check

• Repeatability – place the same weight in the same spot a few times.
• Accuracy – compare the reading with the known mass.
• Eccentricity – the same mass at different positions.
• Linearity – if possible at several points across the range.

Checking a 1,500 kg scale with 1 kg says nothing about what it does at 500 or 1,000 kg. Choose weights that suit the range you work in; the versions are under check weights and test weights.

10. Calibration and adjustment are two different things

Calibration is comparing: the reading is set against known reference weights and the deviation is recorded. Adjustment is correcting: the setting of the instrument is changed to reduce that deviation.

After an adjustment the instrument can be calibrated again, to record how it performs afterwards. The difference from verification – and when you need which – is explained under verification or calibration.

When is a professional check advisable?

Have a scale inspected when:

• the deviation keeps coming back;
• the same weighing does not give the same result twice;
• different spots on the platform clearly differ;
• the instrument has been overloaded, dropped or hit;
• you charge by weight or use it for quality control;
• a periodic calibration is scheduled;
• you cannot determine whether the cause is mechanical, electronic or metrological.

For a verified scale additional requirements apply: after certain repairs or breaking a seal, re-verification may be needed, which is arranged through the manufacturer. What that involves is explained under having a scale re-verified; when a scale must be verified is explained under legal metrology.

Checklist for a scale that deviates

1. Clear the platform completely.
2. Check that the reading is at zero.
3. Check that no tare is active.
4. Check the level and stability.
5. Check that the platform is not touching anything.
6. Remove dirt under and around the platform (see maintaining and cleaning your scale).
7. Let a precise balance warm up first.
8. Weigh the same test weight a few times in succession.
9. Also place that weight at other positions on the platform.
10. If possible, check at several points across the range.
11. If the deviation persists, have the scale calibrated and technically inspected.

Have it checked, calibrated or repaired?

If your scale keeps deviating, we will look at it: in the workshop and, depending on the system, on your premises. Including equipment you did not buy from us. What we do is described under verification, calibration and repair, scale calibration and scale repair. How often calibration makes sense depends on your use; that is explained under how often should a scale be calibrated. What goes into the report is described under the calibration certificate.

Frequently asked questions

Why does my scale keep showing a different weight?

Fluctuating values come from vibrations, draughts, temperature, an unstable surface, mechanical contact with the platform, too little warm-up time or a technical fault. First weigh the same known weight a few times; if the difference remains, it is not the weighing itself.

Can I check myself whether my scale weighs correctly?

Yes. Use a check weight whose mass is known and look at three things: do you get the same result when you place the weight several times, does the value match the known mass, and does it stay the same at other spots on the platform.

Can I use any object as a test weight?

For a rough impression you can, but not for a check that is of any use. For that you need a calibrated or certified weight; otherwise you do not know whether the deviation lies with the scale or with the object.

Does a scale have to be calibrated every year?

Not every application has the same interval. It depends on the use, the risk, the environment, your quality system and the accuracy you need. Checking it yourself in between with a check weight shows a change sooner.

My scale deviates. Does it need to be re-verified?

Not automatically. Calibration, adjustment and legal verification are three different things. For an instrument that falls under the legal rules, re-verification may be needed after certain repairs or after a seal has been broken.

Can a damaged load cell still show a normal weight?

Yes. A load cell does not have to have failed to disturb the measurement. The display can show a plausible value while the repeatability, linearity or corner loading are no longer correct.